Randovania recommends use of SimpleModManager when playing randomizers on Switch.
Start by downloading the latest release of SimpleModManager here: https://github.com/nadrino/SimpleModManager/releases
Copy it to the atmosphere folder on your SD card (or wherever you install homebrew apps) and launch the application.
SimpleModManager main screen
Select Metroid Dread from the games list. You must export a game in Randovania before Dread will appear in the games list!
Metroid Dread screen in SMM
All of your randomized games will be displayed here. You may have multiple randomized games on your Switch, but be careful: only one can be active at a time. Ensure that you deactivate any old games before activating a new one.
Choose the game you want to activate, and press A to apply. You can also deactivate any old games by choosing them and pressing X.
SMM applying the desired mod
When you see Active on exactly one game, everything is finished and you’re ready to launch the game and play.
SMM showing the desired mod as Active